This relates to electronic devices, such as electronic devices that provide bias signals to accessories using an audio jack.
Electronic devices such as cellular telephones, computers, music players, and other devices often contain audio jacks. Accessories such as headsets have mating plugs. A user who desires to use a headset with an electronic device may connect the headset plug into the mating audio jack on the electronic device.
It is often necessary to convey stereo audio signals, microphone signals, and button signals between an electronic devices and a headset connected to the electronic device. In a typical microphone-enabled headset, a bias voltage is applied to the microphone from the electronic device over the microphone line. The microphone in the headset generates a microphone signal when sound is received from the user (i.e., when a user speaks during a telephone call). Microphone amplifier circuitry and analog-to-digital converter circuitry in the cellular telephone can convert microphone signals from the headset into digital signals for subsequent processing.
To convey button signals (e.g., to accommodate additional functionality), some modern microphone-enabled headsets feature a button that, when pressed, shorts the microphone line to ground. Some other headsets also include ultrasonic tone generators that can be used to convey button signals using ultrasonic tones. In these types of arrangements, a headset includes an ultrasonic tone generator that generates ultrasonic tones on the microphone line. The ultrasonic tone generator is typically powered using the bias voltage on the microphone line. Monitoring circuitry in an electronic device to which the headset is connected can detect the momentary grounding of the microphone line and the ultrasonic tones on the microphone line.
Modern headsets typically require that the bias voltage have a sufficient magnitude for proper operation of the headsets. All headsets are, however, susceptible to wear, environmental effects, and other factors that can negatively impact the magnitude of the bias voltage available to circuitry within the headsets. For example, when a headset is drenched in moisture, as may occur when a user wears a headset while sweating, moisture-related shorts may develop in the headset that lower the magnitude of the bias voltage within the headset below a minimum voltage level that is necessary for the headset to operate properly.
It would therefore be desirable to provide electronic devices that provide adjustable bias impedances and adjustable bias voltages to accessories.